We have developed protocols to assess the physiology of isolated leukocytes and leukocytes in blood. These protocols involve the analysis of cell function and the expression of surface receptors indicative of cell activation. We are beginning to characterize the response of leukocytes in septic neonates to administration of G-CSF (Bellis, Sklar, Ohls, manuscript submitted). We are examining cord blood as a function of the period of gestation and neonatal blood from healthy and sick babies.